Cellular wireless networks are widely known in which base stations (BSs) communicate with terminals (also called user equipments (UEs), or subscriber or mobile stations) within range of the BSs. The geographical areas covered by base stations are generally referred to as cells, and typically many BSs are provided in appropriate locations so as to form a network or system covering a wide geographical area more or less seamlessly with adjacent and/or overlapping cells. (In this specification, the terms “system” and “network” are used synonymously except where the context requires otherwise). In each cell, the available bandwidth is divided into individual resource allocations for the user equipments which it serves. Communications in the network comprise downlink communications from the base station to the terminal, and uplink communications from the terminal to the base station. Data to be transmitted in the uplink, in the form of a data stream comprising a sequence of data packets, may be user data or control data and may have different QoS (Quality of Service) requirements, depending on the application or purpose.
Basic LTE Network
One type of cellular wireless network is based upon the set of standards referred to as Long-Term Evolution (LTE). The current version of the standard, Release 11, is also referred to as LTE-A (LTE-Advanced). The network topology in LTE is illustrated in FIG. 1. As can be seen, each terminal 1, called a UE in LTE, connects over a wireless link via a Uu interface to a base station in the form of an enhanced node-B or eNodeB 11. It should be noted that various types of eNodeB are possible. An eNodeB may support one or more cells at different carrier frequencies having differing transmit powers and different antenna configurations, and therefore providing coverage areas (cells) of differing sizes. Multiple eNodeBs deployed in a given geographical area constitute a wireless network called the E-UTRAN (and henceforth generally referred to simply as “the network”). Cells in an LTE network can operate either in a Time Division Duplex, TDD, mode in which the uplink and downlink are separated in time but use the same carrier frequency, or Frequency Division Duplex, FDD, in which the uplink and downlink occur simultaneously at different carrier frequencies.
Each eNodeB 11 in turn is connected by a (usually) wired link using an interface called S1 to higher-level or “core network” entities 101, including a Serving Gateway (S-GW), and a Mobility Management Entity (MME) for managing the system and sending control signalling to other nodes, particularly eNodeBs, in the network. In addition (not shown), a Packet Data Network (PDN) Gateway (P-GW) is present, separately or combined with the S-GW, to exchange data packets with any packet data network including the Internet. Thus, communication is possible between the LTE network and other networks.
Small Cell Network (SCN)
FIG. 1 shows what is sometimes called a “homogeneous network”; that is, a network of base stations in a planned layout and which have similar transmit power levels, antenna patterns, receiver noise floors and similar backhaul connectivity to the core network. Current wireless cellular networks are typically deployed as homogeneous networks using a macro-centric planned process. The locations of the base stations are carefully decided by network planning, and the base station settings are properly configured to maximise the coverage and control the interference between base stations. However, it is widely assumed that future cellular wireless networks will adopt a “heterogeneous network” structure composed of two or more different kinds of cell, also (and henceforth) referred to as a Small Cell Network or SCN.
FIG. 2 depicts a simple SCN. The large ellipse 10 represents the coverage area or footprint of a Macro cell provided by a base station (Macro BS) 11. The smaller ellipses 20, 22 and 24 represent Small cells within the coverage area of Macro cell 10, each having a respective base station (exemplified by Pico BS 21). Here, the Macro cell is a cell providing basic “underlay” coverage in the network of a certain area, and the Small cells are overlaid over the Macro cell, using the same or different carrier frequencies for capacity boosting purposes particularly within so-called “hot spot zones”. A UE 1 is able to communicate both with Macro BS 11 and Pico BS 21 (but not necessarily simultaneously) as indicated by the arrows in the Figure. When a UE starts to use a given cell for its communication, that cell is said to be “activated” for that UE, whether or not the cell is already in use by any other UEs. Incidentally, although the Macro and Small cells are depicted here as being provided by different base stations, this is not essential and the same base station may be responsible for both a Macro cell and a Small cell. For example, a cell operating in a higher frequency band is likely to experience greater pathloss, and thus have shorter range, than one in a lower frequency band; thus the same base station may provide both a lower-frequency Macro cell and a higher-frequency Small cell.
Although only two types of cell are shown in FIG. 2, various types of Small cell may be present in a SCN including (in decreasing order of size), cells similar to current Micro, Pico and Femto cells. Femto and Pico cells can be overlaid on either Macro or Micro cells. Thus, networks can be designed such that the Macro cells provide blanket coverage while the Micro, Pico and/or Femto cells (or Small Cells) provide additional capacity. The envisaged Small Cells may also correspond to a New Carrier Type (NCT) not yet defined in LTE specifications.
Carrier Aggregation (CA)
SCNs will support and enhance various capacity-boosting schemes to be applied to UEs, including so-called Carrier Aggregation (CA) which has been introduced into 3GPP (in the homogeneous network context) since LTE Release 10. Details of CA as applied to LTE are given in the 3GPP standard TS36.300, hereby incorporated by reference.
In CA, two or more Component Carriers (CCs) at different carrier frequencies are aggregated in order to support wider transmission bandwidths up to 100 MHz (made up of a maximum of five CCs each having a bandwidth around their carrier frequency of up to 20 MHz). A UE may simultaneously receive or transmit on one or multiple CCs depending on its capabilities.
Management of connections of UEs to the network, broadcast of system information and establishment of radio bearers is part of Radio Resource Control (RRC). When CA is configured, the UE only has one RRC connection with the network. At RRC connection establishment/re-establishment/handover, one serving cell provides the system information which the UE needs to join the network, and this cell is referred to as the Primary Cell (PCell). All other CCs are called Secondary Cells or SCells. Generally, one carrier corresponds to one cell. In the downlink, the carrier corresponding to the PCell is the Downlink Primary Component Carrier (DL PCC) while in the uplink it is the Uplink Primary Component Carrier (UL PCC). Incidentally, in this specification the terms “carrier” and “cell” are used somewhat interchangeably; it should be borne in mind, however that although different carrier frequencies always imply different cells, the reverse is not necessarily the case: a single carrier frequency can support one or more cells.
Therefore, a UE using CA has a plurality of serving cells, one for each CC, and the CCs may for example correspond to Macro and Small cells in a SCN, such that the same UE may use the Macro cell as its “primary” cell (PCell) and a Small cell as a “secondary” cell (SCell). As well as possibly having different carrier frequencies, the Macro and Small cells may have different bandwidths. Generally, each cell is provided by base station antennas at a single site, but this does not exclude the possibility of one cell being provided by antennas at different sites.
A potential issue with CA in SCNs is that at least some of the CCs are likely to be provided by small base stations similar to existing Home eNodeBs and femtocells, which use broadband internet for their backhaul connectivity to the network, and are therefore liable to incur greater latency (including a greater time taken to exchange information with other base stations) compared with macro cell eNodeBs.
Where the CCs are provided by geographically-separated base stations, these base stations will also generally experience different signal propagation delays from the UE. In order to take advantage of CA in the SCN scenario, Release 11 of LTE provides for multiple uplink Timing Advances (TAs), by which a UE can ensure that its uplink transmissions arrive in synchronization with transmissions from other UEs at the base stations providing the cells. Since the same base station may provide more than one cell, the concept of a Timing Advance Group (TAG) is used to group together carriers with the same TA value. However, various aspects of how CA may be most advantageously applied to the SCN have yet to be determined, as explained later.
Uplink Channels in LTE
Since the embodiments to be described concern buffer status reports (BSRs) in SCNs, some further explanation will be given of the nature of, and need for, these status reports.
An LTE system is a scheduled system in which transmission is organized in “frames” each containing twenty slots, two consecutive slots being referred to as a “subframe”. For each transmission time interval of one or more subframes, a new scheduling decision is taken regarding which UEs are assigned/allocated to which time/frequency/spatial/code resources during this transmission time interval.
Several “channels” for data and signalling are defined at various levels of abstraction within the network. FIG. 3 shows some of the channels defined in LTE-based systems at each of a logical level, transport layer level and physical layer level, each corresponding to a different protocol layer within the well-known OSI model, and the mappings between them. For present purposes, the uplink channels are of particular interest.
In FIG. 3, physical channels defined in the uplink are a Physical Random Access Channel (PRACH), a Physical Uplink Shared Channel (PUSCH), and a Physical Uplink Control Channel (PUCCH). An uplink physical channel corresponds to a set of resources carrying information originating from higher layers. In addition to the uplink channels, uplink signals such as reference signals, primary and secondary synchronization signals are typically defined.
At the transport channel level in FIG. 3, an uplink shared channel UL-SCH maps to the physical channel PUSCH whilst a random access channel RACH maps to the above mentioned PRACH. Incidentally, although FIG. 3 shows logical channels, these define set of logical channel types for different kinds of data transfer services as offered by the MAC, where each logical channel type is defined by what type of information is transferred. It should be noted that in this invention logical channels refer to the logical channels used in the logical channel Prioritisation procedure in the MAC where the actual logical channels are defined by RRC configuration.
The above mentioned 3GPP TS 36.300 provides an overall description of the radio interface protocol architecture used in LTE-based systems and in particular section 5.2 of 3GPP TS 36.300 relates to uplink transmission schemes. The physical channels in the uplink of LTE-based systems are described, for example, in 3GPP TS 36.211, section 5, which is hereby also incorporated by reference.
User data and optionally also higher-level control signalling is carried on the Physical Uplink Shared Channel PUSCH. The physical uplink control channel PUCCH carries uplink control information such as a scheduling request (SR), see below, and a channel quality indicator (CQI) report. As illustrated in FIG. 3, there is a downlink counterpart channel to the PUCCH, which is the Physical Downlink Control Channel (PDCCH) for carrying, in response to the scheduling request, an uplink scheduling grant. Incidentally, in LTE-A there is also an enhanced PDCCH called EPDCCH, which allows coordination among eNodeBs for reducing inter-cell interference.
The uplink scheduling grant also indicates the transmission rate (i.e. modulation and code rate). If PUSCH transmission occurs when the PUCCH would otherwise be transmitted, the control information to be carried on PUCCH may be transmitted on PUSCH along with user data. Simultaneous transmission of PUCCH and PUSCH from the same UE may be supported if enabled by higher layers. The PUCCH may support multiple formats as indicated in 3GPP TS 36.211, section 5.4.
Because transmissions between UE and base station are prone to transmission errors due to interference, a procedure is available for each packet sent in uplink and downlink direction to be acknowledged by the receiver. This is done by sending Hybrid Automatic Repeat Request (HARQ) acknowledgments or non-acknowledgments (ACK/NACK) on control channels. On the downlink, ACK/NACK is sent on a Physical HARQ Indicator Channel (PHICH). On the uplink ACK/NACK is sent on PUCCH.
The Physical Random Access Channel PRACH is used to carry the Random Access Channel (RACH) for accessing the network if the UE does not have any allocated uplink transmission resource. If a scheduling request (SR) is triggered at the UE, for example by arrival of data for transmission on PUSCH, when no PUSCH resources have been allocated to the UE, the SR is transmitted on a dedicated resource for this purpose. If no such resources have been allocated to the UE, the RACH procedure on PRACH is initiated.
Channels in CA
Having outlined some of the more important channels defined in LTE, their relationship to cells/CCs in the CA scenario can now be described using FIG. 4.
As shown in FIG. 4, under current LTE proposals, each PCell can transmit PDCCH to a UE. An SCell may (or may not) provide PDCCH to a UE, depending on UE capabilities; however, uplink data on PUSCH, and BSR and some RACH can be transmitted by a UE having the required capabilities, on both PCell and SCell. Correspondingly there is a separate transport channel UL-SCH for each cell. For LTE up to and including Release 11 the uplink control channel (PUCCH), which supports SR, is only transmitted on the PCell. Similarly, PRACH for scheduling requests is only transmitted on the PCell. However, these restrictions may not apply in future Releases.
If an SCell does not carry PDCCH, this implies that the scheduling information for that cell has to be carried in PDCCH of another cell (typically the PCell)—so called cross-carrier scheduling. The PCell and SCells should have identical or very similar transmission timing which allows, for example, PDCCH on one cell to schedule resources on a different cell, and ACK/NACKs for PDSCH transmissions on SCells to be sent on the PCell. SCells may have different transmission timing at the UE in order to allow for the possibility that the cells are supported by antennas at different geographical sites. A PCell and/or SCells with the same timing would belong to the same TAG (Time Alignment Group). However, because of the tight timing synchronization requirements between PUCCH on the PCell and PDSCH on the SCells, PCells and SCells can be assumed to be controlled by the same eNodeB.
Radio Bearers and Logical Channels
In an OSI-compliant standard such as for LTE, protocol layers defined in the system (i.e. E-UTRAN) include Packet Data Control Protocol (PDCP), Radio Link Control (RLC) and Medium Access Control (MAC). FIG. 5 shows the major uplink functions in the system at each of the PDCP, RLC and MAC protocol layers when employing CA. FIG. 5 shows the case of two component carriers CC1 and CC2, corresponding to a PCell and SCell for example. With respect to a given node (for example an eNodeB or UE), each protocol can be considered to reside in a functional module or “entity” which can be considered separately from protocols in other layers. This allows, among other things, for the use of the concept of “radio bearers”, which provide a kind of tunnel between peer entities in the eNodeB and UE at a given protocol level for user data or control signalling. Establishment and maintenance of radio bearers is another aspect of RRC.
Packets belonging to the same radio bearer get the same end-to-end treatment in the network. A bearer can be characterised by parameters such as “Guaranteed Bit Rate” (GBR) and “non-GBR”. For GBR bearers, the network guarantees a certain bit rate to be available for the bearer at any time. The bearers, both GBR and non-GBR are further characterized by a Maximum Bit Rate (MBR), which limits the maximum rate that the network will provide for the given bearer. In this way it is possible for each radio bearer to provide a certain quality of service, QoS appropriate to a data stream carried on the radio bearer.
In LTE/E-UTRAN, the above mentioned radio bearers (RBs) are defined at various protocol levels including PDCP. There are three kinds of PDCP bearers: SRB (Signalling Radio Bearer) and two kinds of DRB (Dedicated Radio Bearer), AM DRB and UM DRB where AM stands for Acknowledged Mode RLC and UM for Unacknowledged Mode. In E-UTRAN there are only three SRBs—SRB0, SRB1 and SRB2. These are used by control plane protocol to send the packets to the UE. DRBs are used for sending voice and data; as many DRBs are set up as the number of QoS streams or services required by the terminal. When a DRB is set up, a Logical Channel Identity (LCID) will be assigned to this DRB for UL and DL. In this sense, it may be said that one logical channel (LC) conventionally corresponds to one RB.
For the purpose of resource allocation, the logical channels may in turn be assigned to Logical Channel Groups (LCGs), LCs of similar QoS demands (QoS Class Identifier, and/or priority (QCI)) being placed in the same LCG. Each LC is assigned to only one LCG and the assignment of an LC to a LCG may be done on the basis of required quality of service (e.g. priority, delay requirements). The primary motivation for defining LCGs is to reduce the amount of control signalling compared with providing control signalling for each individual LC.
The LC may be assigned to the RB, and LC may be assigned to a LCG, when the RB is set up; however it may be useful to reassign LCs to LCGs for example when a new RB is added or an existing RB is removed.
The MAC layer, apart from managing the above-mentioned hybrid ARQ function, is responsible for mapping of data between the logical channels to transport channels, each of which may carry more than one LC. Transport channels DL-SCH and UL-SCH represent data transfer services offered by the PHY and are defined by how the information is carried, different physical layer modulations and the way they are encoded.
A MAC entity of the eNodeB in an LTE system includes a scheduling function responsible for managing resource scheduling for both uplink and downlink channels, that is, to allocate physical layer resources for the DL-SCH and UL-SCH transport channels. Different schedulers operate for the DL-SCH and UL-SCH. The scheduler should take account of the traffic volume (buffer status) and the QoS requirements of each UE and associated radio bearers, when sharing resources between UEs. Schedulers may assign resources taking account the radio conditions at the UE identified through measurements made at the eNodeB and/or reported by the UE. Likewise, the UE has a MAC entity with a scheduling function for, in accordance with a resource allocation signalled to it by the eNodeB, constructing transport blocks from data for transmission which has arrived in a buffer of the UE.
In the scenario of CA within LTE, as can be seen from FIG. 5, the multi-carrier nature of the physical layer is only exposed to the MAC layer for which a separate UL-SCH is provided by each CC (serving cell) and one HARQ entity is required per serving cell.
The relationship (mapping) between radio bearers, logical channels/LCGs, and transport channels is summarised in FIG. 8, in which the downward direction represents the progression from higher-level to lower-level protocol layers.
Thus, at the top of the protocol stack is an application running on the UE, a camera application or Voice over IP (VoIP) for example. This generates a data stream such as photographs or voice packets to be uploaded to the network. Each data stream has an associated QoS requirement. In the example of a camera application, a “best effort” service would suffice; on the other hand VoIP would have a more stringent QoS requirement to minimise latency. The data stream maps to a radio bearer (DRB) which in turn corresponds to a logical channel. Thus, a data stream may be mapped to one logical channel (although LTE specifications do not prevent mapping a data stream to more than one logical channel), and the logical channel may be assigned to a logical channel group. Transport channels are constructed from the logical channels and transmitted on the uplink physical channels.
Buffer Status Report (BSR) and Scheduling Request (SR)
Based on the current 3GPP standard as set out in TS36.300, measurement reports, including transport volume and measurements of a UE's radio environment, are required to enable the scheduler in the eNodeB to operate in both uplink and downlink. Especially, in the uplink direction, uplink buffer status reports (BSR), including information on buffer status for different logical channels, are needed from the UEs to provide support for QoS-aware packet scheduling by the eNodeB.
That is, in order to schedule uplink transmissions efficiently the network needs to be aware of the amount (volume) of data that the UE needs to transmit, the priority of such data (in the form of data streams of different types and priorities, reflecting different services being provided to the UE), and the uplink channel conditions. In LTE specifications this is provided for by the UE sending BSRs (buffer status reports) along with data transmissions via PUSCH, and by transmission of UL sounding reference signals (SRS).
A BSR indicates the amounts of data buffered at the UE (i.e., ready for transmission on the uplink) with respect to either one or four Logical Channel Groups. In LTE the LCGs are processed with different priorities. Thus, the concept of LCGs allows the BSR to provide information on data amounts categorised by priority. Four LCGs and two formats are used for reporting in the uplink:                A short format for which only the buffer status of one LCG is reported;        A long format for which buffer statuses of all four LCGs are reported.        
The short format is shown in FIG. 6 and the long format is shown in FIG. 7. As indicated in FIG. 6, one specific LCG identified by the first field “LCG ID”, the amount of data being signified by the second field, “Buffer Size”. In the long format BSR shown in FIG. 7, it will be noted that explicit LCG IDs are not used, the LCG concerned being implicit from the relative position of the Buffer Size field within the report. If not all four LCGs are configured then the UE simply reports “0” (zero) for the status of any non-configured LCG.
However, if the UE has no PUSCH resources available, there is no means to send a BSR. In such cases a scheduling request (SR) is triggered in the UE. The SR indicates that the UE needs to be granted UL resources on PUSCH. In some cases the UE may not have an SR resource allocation on PUCCH, and then the RACH procedure is initiated as already mentioned. A BSR supersedes the SR in the sense that when a SR is triggered, it is considered as pending until it is cancelled by transmission of a BSR which contains buffer status up to (and including) the last event that triggered a BSR, or when the UL grant(s) can accommodate all pending data available for transmission.
The BSR and SR protocols are further described in 3GPP TS 36.321, sections 5.4.4, 5.4.5, and 6.1.3, and the SR procedure for a terminal procedure for determining physical uplink control channel assignment is described in 3GPP TS 36.213, section 10, which is also hereby incorporated by reference.
To summarise the foregoing, currently in LTE buffer status reports are used to indicate to the network information about the data ready to be transmitted from the terminal in the uplink. Data streams corresponding to logical channels are associated with logical channel groups, and depending on the arrival and priority of new data, a buffer status report (BSR) may be triggered. If no (PUSCH) resources are available to carry a BSR, a scheduling request (SR) may be transmitted. If no resources for SR are allocated, then a random access (RACH) transmission may take place. In the 3GPP specifications a data stream may be considered to correspond to a radio bearer, which would include both signalling radio bearers (SRB) and data radio bearers (DRB).
In Carrier Aggregation (CA), in addition to the PCell one or more SCells at different carrier frequencies may be configured for a UE. The PCell and SCells should have identical or very similar transmission timing, which allows, for example, PDCCH on one cell to schedule resources on a different cell, and ACK/NACKs for PDSCH transmissions on SCells to be sent on the PCell. Uplink transmissions from the UE via each SCell may have different transmission timings at the UE in order to allow for the possibility that the cells are supported by antennas at different geographical sites. A PCell and/or SCells with the same timing would belong to the same TAG (Time Alignment Group). However, because of the tight timing synchronization requirements between PUCCH on the PCell and PDSCH on the SCells, PCells and SCells can be assumed to be controlled by the same eNodeB.
For a given UE, the SCells may be activated or deactivated by MAC signalling. A terminal may be capable of uplink operation on two (or more) carriers. Therefore in CA, uplink data on PUSCH, and BSR and some RACH can be transmitted by both PCell and SCell. For LTE up to and including Release 11 the uplink control channel (PUCCH), which supports SR, is only transmitted on the PCell. Similarly, PRACH for requesting an UL resource allocation is only transmitted on PCell. PRACH may be transmitted on an SCell, but only for the purpose of determining timing advance. However, these restrictions may not apply in future Releases.
One key scenario for the “Small Cell” concept, which is currently being studied in 3GPP, provides for the possibility of a terminal being supported by both a macro cell and one or more small cells, operating at the same or different carrier frequencies. This has some similarities with CA, but the timing relation between the cells may be less strictly controlled for small cells, and the cells may be controlled by different eNodeBs. It is likely that cells at significantly different frequencies (as envisaged for one of the small cells scenarios) would have different channel conditions and traffic capacities. The small cell carrier may have higher data rate capacity, but less consistent geographical coverage. In addition the backhaul capacity and latency may be different for macro cells and small cells. Therefore, under the Small Cell concept it would be advantageous for the network to be able to control both which data traffic from a given UE is routed via each carrier, and the arrangements for reporting uplink buffer status and scheduling requests, depending on channel conditions, which are likely to vary with time and location.
WO 2011/100673 A1 discloses a wireless communication method, wireless communication network, terminal and base station according to the preamble of each independent claim. When a terminal is connected with multiple eNBs (sites) with one or more UL CCs to each site, one CC per site being defined as an anchor carrier, and one anchor carrier being defined as a primary CC, the corresponding UL anchor carrier associated with the eNB may be used for sending UL control information such as ACK/NACK and channel state information. Alternatively, all UL control information may be sent using the primary CC.
EP 2 391 172 A2 discloses a BSR scheme for a LTE MIMO system in which a single eNB communicates with a UE, using either of the above mentioned BSR formats of FIGS. 6 and 7.
WO 2013/113390 A1 discloses a BSR scheme for a CA system in LTE in which either BSR is transmitted on both PCell and SCell in case both are scheduled, or alternatively the UE sends BSR on the allocated PUSCH as per Release-10 but with BSR cancellation on a CC basis, i.e. BSR for a given serving cell is considered pending until is not transmitted on the corresponding cell.